1. Field of the Invention
The present invention relates to a developing device and a developing method for use in an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or other similar image forming apparatus.
2. Discussion of the Background
In a developing device that uses a one or two-component developer or toner, the developer accommodated in a developer hopper is supplied to a developing roller by agitating and conveying the developer by agitating members. The developer on the developing roller is charged by friction, and a uniform thin layer of the developer is formed on the developing roller by bringing a regulating member into contact with the developer on the developing roller. While rotating the developing roller, which is adjacent to or in contact with an image carrier (i.e., a photoreceptor), the developer adheres to a latent image formed on the image carrier. By this arrangement a toner image is formed on the image carrier. In this developing device, the developer is subjected to a significant mechanical stress both when a thin developer layer is formed on the developing roller by the regulating member and also when the developer is agitated and conveyed by the agitating member.
Generally, a toner is made from a binder resin into which a charge controlling agent (CCA) or a charge controlling resin (CCR) is mixed, and is charged by friction between the toner and a member. To obtain fluidity, inorganic external additives are attached on the periphery of the binder resin. By the attachment of the inorganic external additives, a non-electrostatic adhering force of the toner relative to other toner and members is decreased. The external additives themselves may be charged, thereby contributing to the charge of toner. However, because the hardness of the binder resin is significantly lower than that of metal, the external additives typically become embedded in the binder resin during a period when a developing process is repeated. This phenomenon occurs because the external additives become embedded in the softened binder resin due to the above-described mechanical stress. In this condition, although toner is sufficiently charged, the fluidity of toner decreases due to the embedded external additives, so that toner particles tend to adhere to each other and agglutinate. As a result, the non-electrostatic adhering force of the agglutinated toner relative to a developing roller increases, and the adhering force of toner on the developing roller increases. Further, a charging amount of toner decreases due to the decrease of the covering area of the external additives relative to the binder resin of toner. Consequently, developing performance of toner is decreased, and undesired results such as background fouling, and the decrease of image density, increasingly occur with time. Accordingly, toner easily adheres to members of the developing device, until the members and a developing unit of the developing device are replaced. Generally, the members and the developing unit are disposed of, which negatively impacts the environment.
Further, if the adhering force between toner particles increases due to embedded external additives when a lubricant such as powder of zinc stearate, and polytetrafluoroethylene (PTFE), to be applied to a photoreceptor adheres to the developing roller, an amount of toner adhering to the developing roller significantly decreases when forming a thin developer layer on the developing roller.
In the above-described developing device that supplies developer (toner) to the developing roller from the developer hopper acting as a developer storing section by using agitating and conveying members, the developer hopper or a developer replenishing tank is disposed in the vicinity of the developing roller. This causes the developing device to be designed to reside in a limited volume, and increases the size of a developing unit. Further, operability is degraded at the time of maintenance of members and units of the developing device.
Therefore, it is desirable to provide a developing device, and an image forming apparatus including the developing device that can form a high quality image for a long period of time by minimizing a mechanical deterioration of developer. Further, it is desirable to provide a developing method that can form a high quality image for a long period of time by minimizing a mechanical deterioration of developer.